I own a pair of both boots and rode them last season. I have narrow heels/ankles and a normal width forefoot. The Nitros are definitely for narrower feet and fit pretty snugly without having crank down on the laces. I've found that I don't need to tighten down the laces throughout the day. Maybe once in the morning after a few runs I will tighten them. They are a very stiff boot without the tongue stiffner in them.
I'm not a big fan of the liner lacing mechanism though. I've found it to be a bit difficult to tighten up with any precision. The Nitros have some ventilation holes on top of the toe box and on windy days I could feel the cold air coming in and in the Spring water seemed to find its way in. In either case it was just enough to make it noticeable and not enough to make it miserable. Also, I did have my Nitros warrantied at the end of the season as one of the air bags blew out and one of the lacing loops started to detach from the boot. Replacement was really fast and it is worth noting that the Nitro gear has a two-year warranty.
The Ions are for more normal width feet. However, I added the J bars and they pretty fit well. Not quite as snugly as the Select TLS boots. There wasn't heel lift, but my ankles weren't locked as well as the Nitros. They are very comfortable boots and are considerably softer than the Nitros, which probably explains why they don't feel as snug as the Nitros. There were also less responsive than the Nitros because they are softer. I am also not a fan of the the liner lacing in the Ions. I understand what Burton is doing with it, but I would prefer the liner to have laces. Though I do like the lacing mechanism. However, that is just a personal preference.
Neither boot it the perfect fit for me. For my feet with a normal width forefoot, a narrow ankle, a tall instep and a long arch, they Ions with a custom footbed seem to be most comfortable. Because of a taller instep, my feet tend to fall asleep in the Nitros if they are a smidge overtightened. However, I prefer the snugness and response of the Nitros. And the factory footbed supported my feet well, which is just dumb luck. No cramping as with other boots I have used.
I hope this helps a bit.
Great comparison review!